This invention relates to telephone line holding circuits and more particularly to telephone line holding circuits used to maintain a telephone line on hold while measurements and tests are being performed on that line.
A wide variety of telephone line holding circuits have been developed which enable telephone subscribers to hold active calls when their telephone set is placed in an on-hook condition. These circuits typically employ a fixed impedence which is shunted across the telephone line when it is desired to place the line in a hold condition. The fixed impedance establishes the flow of sufficient current through the line, between the telephone set and the central office, to maintain the line in a hold condition. Examples of circuits such as those described above are disclosed in U.S. Pat. No. 3,895,192, issued July 15, 1975, to Ronald J. Angner and Alexander Feiner; U.S. Pat. No. 4,011,413, issued Mar. 8, 1977, to Robert C. Phillips; and U.S. Pat. No. 4,093,829, issued June 6, 1978, to Robert L. Silberman.
The telephone line holding circuits of this invention include holding circuits employed by technicians and service personnel to perform tests and measurements of the telephone line. In testing the telephone line, the holding circuit is connected across the line at the desired point of measurement. A test tone generator is also connected to the line and provides an AC signal used for testing the frequency characteristics at various points along the length of the line.
To ensure accurate line test results, holding circuits for use in a line test application are required to perform functions which are not required of the conventional hold circuit for use with telephone sets as described above. For example, a holding circuit for use in line testing applications must be capable of maintaining the line in a hold condition over a wide range of central office supply voltages and line resistances. Typically, the line supply voltage is provided from a DC power supply or battery located at the central office. The supply voltage is nominally forty-eight volts, but may vary considerably at different installations. The telephone line resistance, usually referred to as the loop resistance, can also vary greatly depending on the distance between the central office and the point of test on the line. Typically, the loop resistance may vary from a minimum of four hundred ohms corresponding to line test points adjacent the central office to as much as several thousand ohms for long lines where the test point is located a significant distance from the central office. The holding circuit must supply sufficient loop current to maintain the line in a hold condition over the range of line supply voltages and loop resistances described above. By way of example, the Bell Telephone System specifications for holding circuits require that the holding circuit be capable of establishing a loop current of twenty-three milliamps under the conditions of a forty-six volt line supply voltage and a seventeen hundred ohm loop resistance.
Another requirement of holding circuits for use in line testing applications is that the holding circuit must not represent an excessive electrical load to the test tone generator, because excessive loading causes frequency and amplitude distortions which result in inaccurate line measurements.
In addition, when the holding circuit is connected to the line, it must not reduce or clamp the instantaneous voltage amplitude of the test tone applied to the line. If the holding circuit clamps the output voltage of the test tone generator, the tone suffers amplitude distortion which results in line measurement errors. By way of example, typical test tones employed for telephone line testing may have an amplitude level of +10 dbm, and a nine hundred ohm impedance. These values represent a peak voltage amplitude of approximately.+-.4.25 volts across the line at the point at the point of measurement. Accordingly, the holding circuit must not clamp the line to voltages less than this level.
A number of prior art holding circuits have been developed in an effort to meet the requirements stated above. One such circuit employs an inductor which is connected across the line at the measuring point. The inductor is chosen to have sufficiently large inductance so that it represents a high impedance relative to the impedance of the test tone generator at the frequency of measurement. The inductor must also be chosen to have sufficiently low resistance to maintain long lines on hold. However, using such an inductor on lines of relatively short length can result in excessive current flow through the line which may saturate the inductor and cause line measurement errors.
Another type of prior art holding circuit employs an active semiconductor device to shunt the line at the point of measurement in a manner which establishes a constant level of current flow through the line. This type of prior art circuit suffers from several disadvantages. For example, with certain combinations of line resistance and line supply voltage, these types of circuits clamp the line voltage to a value which is below the minimum necessary to perform accurate line measurements. For example, under the conditions of long line length and/or low line supply voltage, it may not be possible to establish a value of line current equivalent to the constant current level of the holding circuit. Under these conditions, prior art holding circuits present a virtual short circuit across the line at the point of measurement. An additional shortcoming of this type of circuit is that the active semiconductor, usually a transistor, can easily be damaged when subjected to high voltage transients which are normally encountered on telephone lines.
It is therefore an object of the present invention to provide a new and improved telephone line holding circuit.
It is another object of the present invention to provide a telephone line holding circuit which can be used in the performance of telephone line measurements and tests without cusing measurement errors.
It is another object of the present invention to provide a telephone line holding circuit for use in the measurement and testing of telephone lines, and which operates over a wide range of line supply voltages and line resistances without introducing measurement errors.
It is still another object of the present invention to provide a telephone line holding circuit which is protected from damage caused by the application of excessive line voltage.